Hair is made up of two main structural components: the medulla and the cortex. The medulla forms the innermost layer of the hair while the cortex forms the middle layer. Understanding the roles of these two structures can provide insight into hair health and function.
An Overview of Hair Structure
Human hair is composed of the following layers, from innermost to outermost:
- Medulla
- Cortex
- Cuticle
The medulla is a soft, unstructured core that runs along the center of the hair shaft. The cortex surrounds the medulla and contains pigment granules that give hair its color. The cuticle is the thin outer covering that protects the inner structures.
The Medulla
The medulla is the innermost layer of human hair. It is a soft, unstructured core that runs along the center of the hair shaft.
The main features of the medulla are:
- It is composed of round cells that contain air spaces.
- It does not play a significant role in strength or structure.
- It is discontinuous and fragmented.
- It may be absent altogether in fine and white hairs.
The medulla cells contain black pigment granules in dark hair. In white hair, the medulla cells lack pigment. The air spaces in the medulla help provide insulation to the hair. However, the medulla is not a continuous structure and does not contribute significantly to the physical strength or elasticity of hair.
Medulla Variability
There is significant variability in the medulla between different hair types:
| Hair Type | Medulla Characteristics |
|---|---|
| Thick or wiry hair | Continuous medulla |
| Fine hair | Little to no medulla |
| Curly hair | Discontinuous medulla |
| Blonde hair | Fragmented medulla |
As shown in the table, thick and wiry hairs tend to have a continuous medulla while fine, curly, and blonde hairs have a more fragmented or absent medulla. The characteristics of the medulla are largely determined by genetics.
The Cortex
The cortex is the middle layer of the hair that surrounds the medulla. This is the thickest layer of the hair fiber and is composed of elongated cortical cells that are bundled together in elongated strands called macrofibrils.
The main features of the cortex are:
- It provides strength and elasticity.
- It contains melanin granules that determine hair color.
- It has a fibrous protein called keratin.
- It has a higher cell density than the medulla.
The cortex contains bundles of keratin and melanin that provide structure, elasticity, and color to hair. The cortex is the thickest part of the hair and is responsible for the mechanical properties of hair.
Cortex Cell Composition
The cortical cells of the cortex are composed of:
- Keratin proteins – fibrous structural proteins that provide strength
- Melanin – pigment granules that determine hair color
- Water – provides moisture
- Lipids – help retain moisture
- Trace elements – small amounts of minerals
The specific composition of cortical cells determines the structure, texture, and color of an individual’s hair. The keratin proteins are the major structural components while melanin is the major pigment.
Cortex Cell Structure
The cortical cells are arranged parallel to the length of the hair fiber. Within the cells, intermediate filaments of keratin are bundled into larger macrofibrils.
| Cortex Structure | Description |
|---|---|
| Intermediate filaments | Fine strands of keratin protein |
| Macrofibrils | Bundles of intermediate filaments |
| Melanin granules | Pigment grains that determine color |
| Cell membrane complex | Outer coating of cortical cells |
The complex organization and interaction between these components allows hair to have great tensile strength and elasticity while also being pigmented.
Differences Between the Medulla and Cortex
The main differences between the medulla and cortex are:
| Medulla | Cortex |
|---|---|
| – Unstructured core | – Structured layer |
| – Has air spaces | – Higher cell density |
| – Discontinuous | – Continuous layer |
| – Minimal role in strength | – Provides strength |
| – May be absent | – Always present |
| – Less organized | – Highly organized |
In summary, the cortex is a highly structured and continuous part of the hair fiber that provides strength and elasticity. The medulla is a loosely organized core that does not significantly contribute to the hair’s physical properties.
Diseases Affecting the Medulla and Cortex
Certain hair conditions and diseases can affect the medulla and cortex, leading to structural abnormalities:
Medulla Conditions
- Trichorrhexis nodosa – the medulla fractures into horizontal pieces, causing the hair to be brittle.
- Monilethrix – the medulla develops periodic constrictions or swellings.
- Pili annulati – air-filled spaces appear within the fragmented medulla.
Cortex Conditions
- Monilethrix – elliptical cavities develop in the cortex, weakening the hair.
- Pili torti – the cortex develops a twisted appearance.
- Menkes kinky hair disease – the cortex has abnormal keratin leading to twisted hair.
These conditions often have a genetic basis and can cause hair fragility or changes in texture. Examining the medulla and cortex under a microscope can help diagnose the specific abnormality.
Examining the Medulla and Cortex
The medulla and cortex can be visualized under a microscope using samples of plucked hair. Some key features that are examined:
Medulla
- Presence or absence
- Continuity vs. fragmentation
- Regularity of structure
- Air spaces
Cortex
- Uniformity of cortex
- Keratinization
- Pigmentation
- Shape and integrity
Normal hair has a fragmented medulla and regular, dense cortex. Abnormalities like swelling, twisting, or breakage indicate disruption of the medulla or cortex.
Medulla and Cortex Changes with Aging
The medulla and cortex undergo characteristic changes with aging:
Medulla Changes
- Becomes more continuous
- Develops longer air spaces
- Accumulates higher levels of metabolites like hydrogen peroxide
Cortex Changes
- Overall diameter decreases
- Cell membranes become more permeable
- Melanin granules aggregate
- Keratin becomes more disorganized
These age-related changes lead to increased porosity, fragility, and graying of hair. The structural deterioration of the medulla and cortex contribute to the thin, dull hair seen in older individuals.
Impact on Hair Care and Cosmetics
Understanding the medulla and cortex aids developers of hair care products and cosmetic treatments. Some applications include:
- Conditioners that penetrate the cortex to improve strength and shine.
- Dyes and bleaches that alter the melanin content of the cortex.
- Perming and straightening processes that rearrange cortical keratin.
- Split end repair creams that bind broken cortical cells.
- Volumizing products that inflate the medulla.
The structure and integrity of the cortex is especially important for maintaining healthy and attractive hair. Cosmetic chemists optimize products to target the cortex and minimize damage.
Conclusion
The medulla and cortex play distinct roles in the hair fiber. The cortex provides physical strength and elasticity for hair due to its organized structure and high keratin content. The medulla is a less structured core layer that contributes minimally to the properties of hair. Diseases, aging, and cosmetic treatments can all impact the morphology and function of these two areas of hair.